This application is based on and incorporates herein by reference Japanese Patent Application No. 11-314552 filed Nov. 5, 1999.
The present invention relates to a control system for variable displacement internal combustion engines, and particularly a control system for operating an internal combustion engine by variably controlling the number of cylinders to which fuel is supplied in accordance with engine operating conditions.
Some conventional internal combustion engines are operated as variable displacement engines by reducing the number of operative cylinders to which fuel is supplied for air-fuel mixture combustion when the engine operates under the low load condition, so that fuel consumption is reduced.
In one type of the variable displacement engines, the intake valve and the exhaust valve of the inoperative cylinder to which no fuel is supplied for the combustion are opened and closed in the intake stroke and the exhaust stroke, respectively, in the same pattern as the intake valves and the exhaust valves of the operative cylinders are opened and closed. Air sucked into the inoperative cylinder through the intake valve is discharged into an exhaust pipe through the exhaust valve without air-fuel mixture combustion in the inoperative cylinder. The amount of nitrogen oxides (NOx) discharged from the exhaust pipe increases due to reaction of air discharged from the inoperative cylinder with nitrogen in high temperature exhaust gas discharged from the operative cylinders.
Under the low engine load condition, the intake pressure in the intake pipe is normally low because a throttle valve is not open so much. The inoperative cylinder suck air from the intake side and pump out the sucked air into the exhaust side at which the pressure is higher than the intake side. Thus, pumping loss increases and fuel consumption improves less, even when the engine operates with less number of operative cylinders.
In another type of the variable displacement engines, the intake valve and the exhaust valve of the inoperative cylinder are maintained closed throughout all strokes for inhibiting the discharge of air into the exhaust pipe and reducing the pumping loss. In this instance, the pressure in the inoperative cylinder does not rise so much due to no air suction. The maximum pressures in the operative cylinders and the inoperative cylinder differ greatly, thus increasing changes in engine output torque and causing engine vibration.
It is therefore an object of the present invention to provide a control system for variable displacement engines which improves fuel consumption without lessening exhaust emission and causing engine vibration.
According to the present invention, an internal combustion engine is operated as a variable displacement engine in such a manner that it is operated in either all cylinder-operated mode or reduced cylinder-operated mode by varying the number of operative cylinders into which fuel is injected. In the reduced cylinder-operated mode, an intake valve of the inoperative cylinder is maintained closed throughout the four strokes, suction, compression, explosion and exhaust, and an exhaust valve of the same is opened only in the suction stroke and the exhaust stroke. The inoperative cylinder sucks high pressure exhaust gas thereinto through the exhaust valve opened in the suction stroke and discharges it therefrom through the exhaust valve opened in the exhaust stroke after the compression stroke and the explosion stroke. Thus, as no air is sucked into and discharged from the inoperative cylinder, discharge of NOx from an exhaust pipe is minimized. Further, as the difference in the pressures between the cylinders of the operative cylinder and the inoperative cylinder is reduced, the engine vibration can be reduced.
Preferably, the intake/exhaust valve control for the inoperative cylinder is started from the suction stroke after one combustion cycle of the inoperative cylinder from the injection of fuel into the inoperative cylinder, when the engine is switched from the all cylinder-operated mode to the reduced cylinder-operated mode. The intake/exhaust valve control for the operative cylinder is terminated from the suction stroke after the injection of fuel into the inoperative cylinder, when the engine is switched from the reduced cylinder-operated mode to the all cylinder-operated mode.